Super Mario World/Enemies/Reznor

Reznors are monsters that look like triceratops and hang out at the end of fortress levels. The only way to beat them is to knock them from underneath their platform.

Mario fighting four Reznors

After you knock off two Reznors, the platform underneath you will start to disappear, giving way to the lava lake beneath. Jump up to one of the platforms the Reznor was on for safety. From there, you will need to jump up to hit the rest of the Reznors. However, if you're quick, you can sometimes defeat all four Reznors without having to jump on the moving platforms.

Trivia

It is often said that Reznor is a reference to Trent Reznor.